The field of free-form fabrication has seen many important recent advances in the fabrication of articles directly from computer controlled databases. These advances, many of which are in the field of rapid prototyping of articles such as prototype parts and mold dies, have greatly reduced the time and expense required to fabricate articles, particularly in contrast to conventional machining processes in which a block of material, such as a metal, is machined according to engineering drawings.
One example of a modern rapid prototyping technology is a selective laser sintering process. According to this technology, articles are produced in layer-wise fashion from a laser-fusible powder that is dispensed one layer at a time. The powder is sintered, by the application of laser energy that is directed in raster-scan fashion to portions of the powder layer corresponding to a cross section of the article. After the sintering of the powder on one particular layer, an additional layer of powder is dispensed, and the process repeated, with sintering taking place between the current layer and the previously laid layers until the article is complete. Detailed descriptions of the selective laser sintering technology may be found in U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,017,753, U.S. Pat. No. 5,076,869 and U.S. Pat. No. 4,944,817, the entire disclosures of which are incorporated by reference herein. Similarly, a detailed description of the use of selective laser melting technology may be found in U.S. patent application Ser. No. 10/704,270, filed on Nov. 7, 2003, now U.S. Pat. No. 7,537,664 (“the '664 patent”), the disclosure of which is incorporated by reference herein. The selective laser melting and sintering technologies have enabled the direct manufacture of solid or porous three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including wax, metal powders with binders, polycarbonate, nylon, other plastics and composite materials, such as polymer-coated metals and ceramics.
The invention claimed in the '664 patent was the first of many inventions assigned to Howmedica Osteonics Corporation, who has been a pioneer in porous surface and porous structure formation, specifically for use in orthopedics. For instance, other applications in this area, such as U.S. patent application Ser. No. 11/027,421 filed on Dec. 30, 2004 (“the '421 Application”), and U.S. patent application Ser. No. 12/846,327 filed on Jul. 29, 2010 (“the '327 Application”), the entire disclosures of which are hereby incorporated by reference herein, have taught the generation of a population of porous geometry, a mathematical representation of the portion of geometry of the porous structure to be built within a region defined by predetermined unit cells or imaginary volumes that are organized to fill and form a predetermined build geometry, which may be used to produce a near net-shape of an intended porous tissue in-growth structure. The predetermined build geometry, or overall computer-aided design (CAD) geometry, may refer to the mathematical or pictorial representation (such as that on a computer display) of the extent or outer boundary of an intended physical structure to be manufactured. In the case of physical components that include both porous material and solid material, the build geometry may be an assembly of solid and porous CAD volumes that model the outer boundaries of the respective solid and porous materials intended to be manufactured. Furthermore, these applications teach the randomization of the position of interconnected nodes, or points of intersection between two struts or between a strut and a substrate, that define each of the porous geometries while maintaining the interconnectivity between the nodes. As previously taught, such randomization may accomplished by changing the coordinate positions of the nodes in the x, y, and z directions of a Cartesian coordinate system, to new positions based on a defined mathematical function. To achieve a required external shape for a device being created, these references have taught the truncation or removal of struts forming the unit cells at the outer surface. Such truncation helps to achieve the near-net shape of the intended structure, but truncated or clipped struts may, in some instances, create a situation where the porous geometries are un-supported by the underlying structures. These truncated struts may present a potential site for the generation of debris as protruding struts may fracture. Thus, a new method is needed to create build geometries having surfaces that are more robust and less likely to form debris.